Phosphate slurry mining process



United States Patent 3,359,037 PHOSPHATE SLURRY MINING PROCESS RichardL. Every and Ralph C. Hughes, Ponca City, Okla., assignors toContinental Oil Company, Ponca City, Okla, a corporation of Delaware NoDrawing. Filed Mar. 30, 1966, Ser. No. 538,572 6 Claims. (Cl. 2994)ABSTRACT OF THE DISCLOSURE Phosphate matrix comprising an agglomerate ofdiscrete phosphatic particles cemented together by nonphosphaticmaterials is mined by contacting the deposit with a mineral acid tophysically disintegrate the deposit, followed by withdrawing asuspension of particles in acid.

Related application This application is a continuation-in-part of US.Ser. No. 491,898, filed Sept. 30, 1965.

Disclosure This invention relates to a method of mining phosphate rock.In one aspect, the invention relates to removal of phosphate values frombeneath an overburden, without the necessity of removing the overburden,by converting the values into a slurried condition in situ. In anotheraspect, the invention relates to a method for breaking down the physicalstructure of a phosphate-bearing matrix by leaching with a dilutemineral acid.

Phosphate ore or matrix occurs in deposits located in various spotsaround the world, and in differing conditions. For instance, somephosphate deposits form outcrops to the surface, While more commonlythey are buried beneath varying amounts of overburden which itself haslittle or no value. The phosphate values have, in the past, beenrecovered by various techniques. Most common is removal of theoverburden physically by such as draglines, followed by mining thematrix. This technique requires a substantial investment in earth-movingequipment, and becomes uneconomical where the deposlt underlies a thickoverburden. Another method involves vertical shaft mining, withunderground recovery of the matrix by manual and machine labor. Again,substantial equipment investment is required. A third technique whichhas been considered is the use of impingement water, applied throughnozzles in a well bore penetrating the phosphate stratum, to place thematrix in a slurry condition so it can be pumped from the formation.Such a method is described, for instance, in US. Patent 931,057 toGoldsmith, issued August 17, 1909. This method suffers from a lowrecovery of matrix from the formation due to limitations in the radiusin which an impingement stream 15 effective, estimated in the order ofonly 63% of the total matrix, and from a relatively large investment forthe large-diameter boreholes required for the nozzles.

It is an object of this invention to provide method for recoveringphosphate matrix from a formation which is under a thick overburden inan economical manner. Another object of this invention is to providemethod for recovering phosphate matrix without investment in heavyearth-moving equipment. Another object of the invention is to providemethod for recovering a high percentage of the total phosphate matrixcontained beneath a thick overburden.

Other aspects, objects and the several advantages of this invention willbecome apparent upon study of this disclosure and the appended claims.

According to this invention, it has been discovered that phosphatematrix can be physically broken down into discrete, relatively fineparticles of solid values by exposing the phosphate stratum to contactwith a dilute mineral acid. The resulting fine particles of phosphatevalues can then be recovered from the formation by slurrying in a liquidwhich is pumped above ground for further beneficiation. Phosphate matrixcan be considered as an agglomerate of discrete phosphatic particlescemented together by nonphosphatic materials, and the present inventionsucceeds primarily by chemically attacking the cementing material; anychemical attack on the phosphatic particles is incidental in the processof this invention.

Exposure of the phosphatic stratum to attack by dilute acid can beeffected in several manners. The presently preferred method of thisinvention comprises penetrating the phosphate stratum with a pluralityof well bores, producing a substantially horizontal planar fracture inthe pay stratum, and passing the dilute acid solution into the fracturein contact with the phosphatic stratum. After the acid solution has hadsuflicient time to attack the formation, a slurrying fluid such as wateror additional dilute acid is passed into a least one of the well boresat suflicient velocity to form a slurry with the loosened phosphaticparticles, and the slurry is recovered by way of another of the wellbores. The fracture can be immediately adjacent either the lower or theupper boundary of the phosphatic zone, or can be intermediate theseboundaries at some level within the zone. It is also contemplated that aplurality of fractures at different levels throughout the stratum can beproduced and, if desired, held open by a suitable propping agent asknown in the art. These fractures can then be produced eithersimultaneously or sequentially, the advantage in the latter methodresiding in the fact that low volumes of slurrying fluid are required tomaintain slurrying velocity up until such time as the various fracturesbecome enlarged so as to merge with each other.

In addition to the fracture method just discussed, other methods areavailable for subjecting the formation to acid attack. When the payformation is of sufficient permeability, the dilute acid solution can beintroduced therein under pressure so as to penetrate by pressure andcapillary action. Naturally-occurring fissures in the pay formation canalso be used as passageways through which to spread the acid.

Regardless of the manner in which the dilute acid is put into contactwith the formation, the next step comprises maintaining the acid incontact for a time suflicient to break down the structure of a desiredportion of the matrix. The exposure time required depends on severalfactors, but primarily on the tightness of the formation and thechemical nature of the cementing material. Other factors include thetype and strength of acid used, and the formation temperature. Simplelaboratory tests can be used to determine the desirable exposure time,e.g., a core sample of the matrix is subjected to the acid, and the timeto produce a desired loosening is measured. Typically, this initialexposure time will vary between about a half hour and a day, and moretypically from about one to about six hours.

Among the acids suitable for practice of the present invention aresulfuric, phosphoric, nitric and hydrochloric. Hydrochloricacid ispresently less preferred when the phosphatic product is destined forfertilizer use, since fertilizers generally require as low a chlorideion content as possible. Nitric acid is at present one of the moreexpensive per unit of acid strength and, for this reason, is lesspreferred. A typical material with which the phosphatic grains arecemented together comprises in part dolomite and calcite in varyingproportions, and it has been found that sulfuric and phosphoric acidshave different effects on this material. Sulfuric acid seems to attackthe calcite only, while phosphoric attacks both the dolomite and thecalcite. However, the overall effect on loosening of the granulesappears to be very similar in both instances. The point to be observedhere is that various phosphatic deposits react differently to treatmentwith different acids; simple laboratory experiment serves to determinewhich acid is most effective on a given matrix type. This concept ofdifferential effect of various acids on underground formations can alsobe useful in the acidizing treatment of e.g. oil wells.

Mixtures of these acids can also be used to advantage. One source of amixed dilute sulfuric-phosphoric acid is spent acid from the wet processof making phosphoric acid. Acid concentration can be varied withinrather broad limits, although the acid strength should not be so high asto allow appreciable reaction with the phosphatic portion of the matrixwhile it is still underground. Acid (3011-. centrations as low as e.g.0.1 weight percent sulfuric and even lower are useful in breaking up thestratum, and concentrations of 10 weight percent and more are alsocontemplated. A preferred range of acid concentration is between about0.1 and about 5 weight percent.

Various additives can be used in the dilute acid. For example corrosioninhibitors can be used to reduce attack of the acid solution uponequipment used to introduce the solution to and recover the slurry fromthe formation. Foam inhibitors or foaming agents can also be used; thechoice of which of these to use, if either, will depend upon the natureof the phosphate deposit, e.g., its organic content, and upon how muchbeneficiation of the matrix is desired in situ. This latter factorarises from the observation that treatment of phosphate values inaccordance with the method of this invention results in considerablefoam formation, particularly with higher acid concentrations. Onewell-known method of beneficiating phosphatic ores is froth flotation,whereby non-value materials such as silica are separated from phosphaticvalues by becoming either the sink or the float portion of a flotationprocess and, as stated, such a beneficiation can be at least partiallyeffected within the formation by practice of this invention. Further, itis noted that there is an evolution of gases during treatment of thematrix with dilute acid in accordance with this invention. This factaids in recovering the phosphate particles slurry from the recovery wellbecause of the gas-lift effect provided.

The present invention is also advantageous in furnishing a phosphaticproduct from the recovery well which is readily amenable to furtherprocessing. The surface of the particles is to an extent pre-digested orat least conditioned for the subsequent treatment with acid whichtypically initiates the processing of phosphate rock.

It has further been discovered that the gas, primarly carbon dioxide,evolved by this process can be utilized to control the shape of thecavern being mined, i.e., to control the direction of attack of themining solution on the formation walls. This effect is related to thesolubility of carbon dioxide in the particular mining solution beingused. When sufficient pressure is applied to the mining solution, duringits attack on the formation, to prevent evolution of gas bubbles bymaintaining the gas in solution, the acid mining solution exhibits asubstantially higher rate of attack on the phosphatic formation thanwhen the pressure is sufficiently low to allow gas bubble formation.This effect is utilized in the following manner when, for example, it isdesired to selectively mine certain ore zones. Application of a highpressure, i.e., sufficient to prevent gas bubble evolution, results inthe formation being attacked in a generally upward direction, while apressure low enough to permit gas evolution results in the cavernspreading more in a lateral direction. Control over cavern size andshape can also be exercised by the proper placement of acid injectionand slurry recovery points. Thus, by gradually raising these points, theless dense injection acid flows over the top of the heavier slurry andattacks the roof matrix. As

the roof is attacked, it falls down into the lower slurry and forces theacid even higher, resulting in a gradual increase in the cavern height.Thus, by proper control of acid concentration, injection and removalpoint locations, and applied pressure, the cavern size and shape can becontrolled and selective mining of various zones is achieved.

The method of the present invention can be effected in either a batch ora continuous manner, although the latter is presently preferred. Whenaccomplished batchwise, the formation is exposed to the dilute acidsolution for the desired contact time, and then the mass of acid andloosened particles are displaced with additional dilute acid or anotherdisplacing fluid such as water, at a velocity sufficient to produce aslurry of the loosened particles. The stratum is then again filled withfresh dilute acid, and the exposure and flushing procedures arerepeated. When effected continuously, the dilute acid is placed intocontact with the formataion for the desired initial exposure time, andthen additional dilute acid is continuously passed into the formation ata rate sufficient to maintain the desired slurry consistency. Uponobtaining the product slurry above ground, a simple settling operationsuffices to settle out the product phosphate, and the dilute acid can berecycled to the formation after being re-fortified as necessary. In alarge phosphate field, a plurality of wells can be used in a mannersimilar to secondary recovery of oil, e.g., in the so-called five-spot,inverted five-spot, nine-spot, etc., patterns. When production from afield ceases to be economical, the last amount of product can berecovered by passing in a chaser fluid, and then the resulting caverncan be used to dispose of the large volume of wastes typicallyassociated with e.g. phosphoric acid manufacture, such as slimes andrailings. This procedure has the additional advantage of providingpermanent support for the overburden.

Further understanding of the invention will be gained by considerationof the following examples.

EXAMPLE 1 A phosphate deposit occurs which is about 45 feet thick on theaverage, and is underneath an average of about feet of overburden. Thephosphatic deposit is of about 33% BPL and 43% insolubles. The formationis pentrated by two well bores about 75 feet apart, and a substantiallyhorizontal planar fracture is produced between these wells at the bottomof the phosphatic stratum. Sulfuric acid of about 3 weight percent inwater is pumped through one of the well bores so as to substan tiallyfill the fracture. After allowing a reaction time of about 2 hours, acidsolution is continuously pumped into one of the well bores and a slurryof phosphatic particles is recovered from the other.

EXAMPLE 2 A vessel of about 1 foot internal diameter by one foot highwas charged with tightly packed phosphate or like that described inExample 1. A passage about Ar-inch in diameter was made horizontally ona diameter of the vessel. The passage was filled with 1% sulfuric acidand allowed to sit for about one hour, after which time acid of the samestrength was passed continuously through the fracture at a rate of about30 -ml./hr. The slurry product levelled off after about 4 hours of suchoperation at a relatively constant 45 weight percent solids.

EXAMPLE 3 The solubility of carbon dioxide in dilute sulfuric acid hasbeen determined as having a linear relationship between acidconcentration and applied pressure. For an acid strength of about 0.4weight percent, saturation is achieved at atmospheric pressure, whilewith an acid strength of 3 weight percent, saturation requires apressure of about 138 p.s.i.g. When the process of Example 1, wherein 3weight percent sulfuric acid is used, is operated with a pressure in thecavern of about 150 p.s.i.g., the cavern remains liquid-full in spite ofthe evolution of carbon dioxide, and the cavern growth progresses in anessentially upward direction. In contrast, when the same acid strengthis used, but operating under a pressure of about 120 p.s.i.g., carbondioxide is evolved in gaseous form, and cavern growth is primarilylaterally.

Having thus described the invention by providing specific examplesthereof, it is to be understood that no undue limitations orrestrictions are to be drawn by reason thereof and that many variationsand modifications are within the scope of the invention.

What is claimed is:

1. The method of removing phosphatic mineral from an underground depositthereof, said deposit comprising an agglomerate of discrete phosphaticparticles cemented together by nonphosphatic materials, which comprisespassing into contact with said deposit in situ a solution of a mineralacid, maintaining said contact for a time sufiicient to effect physicaldisintegration of a portion of said nonphosphatic material, andwithdrawing acid from said contact at a velocity sufficient to eifectsuspension of solid phosphatic particles therein.

2. The method of claim 1 wherein said acid is selected from the groupconsisting of sulfuric, phosphoric, nitric, and hydrochloric.

3. The method of claim 2 wherein said acid has a concentration, prior tosaid contacting, between about 0.1 and about 5 weight percent.

4. The method of recovering phosphatic mineral from an undergrounddeposit thereof, said deposit comprising an agglomerate of discretephosphatic particles cemented together by nonphosphatic materials, whichcomprises passing a Well bore from the surface of the earth into saiddeposit, passing a second well bore from the surface of the earth intosaid deposit, exerting fluid pressure by Way of one of said well boresinto said deposit so as to cause a fracture therein extending betweensaid well bores, introducing a dilute acid solution by way of one ofsaid well bores into said fracture, subsequently passing additionaldilute acid solution by way of one of said well bores into saidformation at a rate sufiicient to effect suspension of phosphatic solidstherein, and recovering by way of a second of said well bores a productcomprising discrete particulate phosphatic solids suspended in acid.

5. The method of claim 4 wherein said acid is selected from the groupconsisting of sulfuric, phosphoric, nitric and hydrochloric.

6. The method of claim 5 wherein said acid has a concentration, prior tosaid contacting, between about 0.1 and about 5 weight percent.

References Cited UNITED STATES PATENTS 3,070,361 12/1962 Pew W 299173,086,760 4/1963 Bays 299-5X 3,097,922 7/1963 Beetz 23-165 3,186,7936/1965 Gillis et a1. 23-16S 3,278,233 10/1966 Hurd et al. 299-41,690,446 11/1928 Grant.

2,251,916 8/1941 Cross.

ERNEST R. PURSER, Primary Examiner.

1. THE METHOD OF REMOVING PHOSPHATIC MATERIAL FROM AN UNDERGROUNDDEPOSIT THEREOF, SAID DEPOSIT COMPRISING AN AGGLOMERATE OF DISCRETEPHOSPHATIC PARTICLES CEMENTED TOGETHER BY NONPHOSPHATIC MATERIALS, WHICHCOMPRISES PASSING INTO CONTACT WITH SAID DEPOSIT IN SITU A SOLUTION OF AMINERAL ACID, MAINTAINING SAID CONTACT FOR A TIME SUFFICIENT TO EFFECTPHYSICAL DISINTEGRATION OF A PORTION OF SAID NONPHOSPHATIC MATERIAL, ANDWITHDRAWING ACID FROM SAID CONTACT AT A VELOCITY SUFFICIENT TO EFFECTSUSPENSION OF SOLID PHOSPHATIC PARTICLES THEREIN.